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Half-life and changing isotope risk
Table 3 shows the half-lives of two of the radioactive isotopes that contaminated the environment.
A soil sample was taken from the area around Chernobyl in 1986
The soil sample was contaminated with equal amounts of caesium–137 and iodine–131
Explain how the risk linked to each isotope has changed between 1986 and 2018
Both isotopes emit the same type of radiation.
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Marking points
- 1 State that the risk or activity from iodine-131 has decreased by a large amount.
- 2 Explain that this is because iodine-131 has a short half-life of 8 days, so many half-lives have passed.
- 3 State that the risk or activity from caesium-137 has not decreased by much.
- 4 Explain that this is because caesium-137 has a long half-life of 30 years, so only about one half-life has passed.
Full-mark answer
The risk from iodine-131 has decreased by a large amount because its half-life is only 8 days, so many half-lives have passed since 1986. The risk from caesium-137 has not decreased by much because its half-life is 30 years, so only about one half-life has passed by 2018.
Why this answer loses marks
Both risks have decreased by the same amount because they started equal and emit the same radiation.
The risks do not decrease by the same amount because the isotopes have very different half-lives.